Abstract: The underground stress-relief in rock cores in boring has been studied by using the photoelastic, electrometric and computerized techniques. The range and distribution of residual stress have been found. According to the energy principle, one-dimensional and two-dimensional equations of underground stress were derived on the basis of the above data. Another one-dimensional equation of underground stress has been proposed by using a testing method. Based on the three characteristic parameters of disced rock cores, i. e. the diameter （D）, the thickness （ t ） and the difference between the long and short axes （ΔD）, the magnitude of underground stress can be calculated, which is much simpler than the measurement in field.Various calculating equations of underground stress at home and abroad are studied. The proposed two-dimensional equation was applied to the data of Ertan Hydroelectric Power Station. The underground stress was about 700kg/cm2, which was close to 650kg/cm2, measured in field by the Institute of Rock and Soil Mechanics, Academia Sinica, Wuhan.